This invention relates to a method of sensing an abnormal condition in a robot control apparatus and, more particularly, to a method for assuring safe operation upon sensing an abnormal condition such as an overload acting upon a robot.
The rising cost of labor in the industrialized world has given rise to a demand for labor-saving measures and for improvements in work procedures. Industrial robots have been put into use to meet this demand and have been very successful in doing so. Particularly outstanding results have been achieved in plants having numerous machine tools where industrial robots are used to perform such simple tasks as changing the workpieces and tools associated with each machine tool. The demand for such robots has grown yearly. These industrial robots have a so-called "lay-back" capability, in which the robots are taught their tasks or services and then caused to perform these tasks as necessary. Specifically, a teaching box is used to enter the tasks that are to be performed, these tasks constituting robot instruction data which is stored beforehand in a memory located within the associated control unit. Whenever a request for a particular service arrives from the machine tool, a series of the above robot instruction data is read out of the memory sequentially, and the robot responds to the instructions by servicing the machine tool as required.
The robot instruction data comprises positional information regarding the point at which a service is to be performed, robot operating speed, and service codes which instruct how the robot hand is to be controlled at the abovementioned point and which instruct also the exchange of signals with the machine tool side. In general, the teaching operation mentioned above proceeds in the following order: (1) establish the memory addresses of the locations at which the items of robot instruction data are to be stored; (2) position the robot arm by a jog feed operation (that is, by moving the arm manually); (3) set the positional information regarding the servicing point and set the value of the speed command; and (4) set the robot service codes. A series of robot operations related to a machine tool is taught by repeating the aforesaid steps (1) through (4). Accordingly, as long as there are no problems with the mechanism or control system of the robot, and after the robot arm has been positioned at the predetermined operating speed, the robot will respond to the instruction data to execute such services as workpiece and/or tool exchange, cleaning of machining scraps, manipulation of the robot hand and the like.
There are cases, however, where the robot may malfunction or where an abnormal condition may develop in the associated peripheral equipment. When such is the case the robot may grasp an extremely heavy workpiece, the hand of the robot may close upon itself with the intent to grasp a workpiece, or the robot may attempt to lift a workpiece together with the associated jig. It may even press against the machine tool. The occurrence of such overload malfunctions can result in damage to the robot or machine tool or both. The conventional systems have not been designed to deal adequately with the foregoing problems and therefore do not assure a suitable degree of safety.